1. Field of the Invention
The present invention generally relates to a method and apparatus for preventing counterfeiting of digitally encoded media such as audio/visual and computer software compact disks (CDS) and, more particularly, to a watermarking technique with no perceptible trace so that the original data set is not modified and the authentication encoding is not obvious.
2. Background Description
Counterfeiting costs billions of dollars yearly to compact disk companies, software companies and other industries around the world. Several methods have been proposed to fight against counterfeiting. In application Ser. No. 09/060,026, a coded message is associated with the combination of the significant content of the disk and a serial number on the disk. This coded message is hidden using some least significant bits of the recording. However, musicians usually consider the standard 16-bit technology used to digitize musical signals for compact disk recording insufficient to fully render the analog music quality. As a consequence, sacrificing a few bits, or even some of the least significant bits, is considered unacceptable by music producers. It is possible to intertwine the musical signal with a coded signal not made audible by the compact disk player but, in most obvious implementations at least, this would require the use of special disk readers, a solution which is clearly unappealing from a commercial point of view. It is also possible to choose the bits carrying the authentication code according to some model for musical perception in order to minimize the audible effects of changing the audio data, but this cannot be expected to be as good as keeping the full sixteen bits.
In addition to music data files, other types of data files would better be recorded without any change of the significant content. In the case where one records something other than music, alternatives to the present invention are easily provided. However, the present invention still has a significant advantage that it is not obvious that any encoding has been used (which is possibly desirable in some contexts) and that neither a special reader nor special software is necessary at the reading end, except to extract the watermark. Thus, besides music, another important application of the present invention is provided by data such as computer program code where the data is often needed with full precision and, if the format is fixed, there is no obvious space usable to embed a protecting code to guarantee that the data have not been modified.
Thus, the main problem solved by the present invention stems from the fact that methods, such as digital watermarks in the prior art, cannot be used for several types of applications, since they modify the original data set.
It is therefore an object of the present invention to provide a method and apparatus which permits recognition whether a recording is original and/or if it has been performed by the legitimate originator.
It is another object of the invention to provide a way to authenticate a digital recording where no significant bit of the recording can be modified for purposes of the authentication.
It is a further object of the invention to provide a watermark for digital recordings to protect the recordings from counterfeiting but which does not require special apparatus for reading the recordings.
According to the invention, the basic principle is to hide all of the authentication data in the error correcting code (ECC) of the digital recording. The method of the invention can be used both to guarantee originality and to recognize counterfeiting. In the latter case, a serial number may be attached to any recording and serves, together with the significant content of the recording, to create the protecting code. A counterfeiter can only produce legitimate pairings between the serial number and the encoding by copying originals and can only duplicate as many unique, verifiable such pairs as he has access to. Depending on the size of the watermark, the probability of error in the recovered watermark due to read/write errors can be reduced by means of a second level of error correction coding.